Annu. Rev. Astron. Astrophys. 1992. 30: 429-456 Copyright © 1992 by Annual Reviews. All rights reserved

4.4 Conclusion

The probable picture that is forming from current observations is that the observed spectra and number density of quasars and known AGN cannot account for the 3-300 keV XRB. Sources clustered like quasars can at most account for 70% of it. The most precise results have been obtained from deep imaging studies in the soft X-ray band where an excess intensity has been measured in the XRB (Figure 4). The straightforward interpretation is that the observed quasars (down to say m_B ~ 24) produce this excess intensity and contribute little to the 3-300 keV XRB, which remains a puzzle. Although it is possible that quasar spectra are complex and flatten above a few keV, their number density appears to be too low to be consistent with the total number of sources required by fluctuation analyses. Whatever sources are involved, their individual luminosities must be less than that of quasars.